Optical lithography is a key element in integrated circuit (IC) production. It involves passing radiation (light) through a mask of a circuit design and projecting it onto a substrate, commonly a silicon wafer. The light exposes special photoresist chemicals on the surface of the wafer which is used to protect unetched circuit details. Integrated circuit feature resolution is directly related to the wavelength of the radiation. The demand for ever smaller IC features is driving the development of illumination sources that produce radiation having ever smaller wavelengths. Extreme ultraviolet light (EUV) has shorter wavelengths than visible and UV light and can therefore be used to resolve smaller and more numerous features.
Extreme ultraviolet lithography is a promising technology for resolving feature size of 50 nm and below. There are many problems in order to realize EUV lithography and the most serious problem is to develop the EUV radiation source. An EUV source with a collectable radiation power of 50 W to 150 W at over 5 kHz in the spectral range of 13–14 nm will be required to achieve requirements for high volume manufacturing of 300 mm wafers.
Electrical discharge gas plasma devices (EUV lamps) are under investigation as promising EUV sources. The principle consists of heating up certain materials into a plasma to such a level that the material emits EUV radiation. Potential source materials which emit EUV radiation at excited energy levels include xenon, oxygen, and lithium. The aim is to produce as many photons as possible in the required wavelength range. A pulsed discharge of electrically stored energy across a gap between a cathode and an anode is used in the presence of the gas for the creation of plasma with temperatures of several 100,000 C. This plasma emits thermal radiation in the spectral range of around 10 nm to 20 nm.
FIG. 1 is a cross-sectional view of one possible configuration of an electrical discharge gas plasma head 10 capable of producing an EUV-emitting plasma 20. The plasma head 10 comprises a plurality of closely positioned electrodes, in this example represented as a cathode 12 and anode 14, separated by an insulator base 16 or ring separator.
The area between the cathode 12 and anode 14 is filled with an ionizing gas 22. A plasma discharge 17 initiated near the base 19 travels along the cathode 12 and anode 14 through self-induced electromagnetic forces. Upon reaching the cathode tip 18 and anode tip 15, the discharge 17 compresses upon itself densifying, heating, and emitting EUV excitations. Other electrode/insulator geometry is possible but all share the property of producing pinched plasma in close proximity to one of more surfaces of the plasma head.
In operation, a tremendous heat load, on the order of 5 kW/cm2, is experienced by the components of the plasma head 10. The plasma-facing components (PFCs) include: an inner cathode surface 11 of the cathode 12, an outer anode surface 13 of the anode 14, and exposed insulator base surfaces 19 of the insulator base 16. Regardless of the specific component configuration and arrangement, there will be at least some PFCs that are susceptible to the effects of the operation of the plasma head 10.
The PFCs are commonly only a few millimeters from the plasma 20 and in an erosive environment that quickly damages the PFC's. This erosion severely effects performance, lifetime and reliability of the discharge head 10. In particular, the anode 14 tends to erode more quickly than the cathode 12, which puts severe limitations on the lifetime of the discharge head 10 as well as producing debris that can impinge upon and harm the other components of the plasma head and overall system, as well as harm exposed targets being illuminated. provide little protection, at best, for the PFCs. One attempt incorporated internal cooling channels within the structure of the cathode 12 which helps to keep the bulk structure of the cathode 12 from overheating, but provides little protection for the plasma facing inner cathode surface 11 of the cathode 12 from erosion and thermal damage, and provide nil protection for the outer anode surface 13.
The cathode 12 and anode 14 are commonly made from refractory metals, such as tungsten or molybdenum which are more resistant to the effects of extreme heat. These materials are expensive, difficult to machine, and are prone to cracking when structurally loaded under sever heating conditions. These materials, none the less, erode over time in this environment.
The insulator components, namely the insulator base 16, comprise various ceramic materials, all of which suffer to some extent, from thermal cracking and erosion in these environments.
In order for the electric discharge plasma EUV sources to meet commercial requirements and demands, including reliability and productivity, lifetime-extending improvements will have to be made for the components of the discharge head 10.